Fix VQE result, add ansatz

[ElePT]

Summary

On #8702 and #8640, the eigenstate was removed from the eigen-result classes, which has affected certain workflows in qiskit-nature (@mrossinek).

This PR adds the ansatz to the eigensolver/minimum_eigensolver results so that the eigenstates can be retrieved from the result directly (in other words, making the result self-contained). The code would look like:

sampler = Sampler()
job = sampler.run(vqe_result.ansatz, vqe_result.optimal_point)
eigenstate = job.result().quasi_dists[0]

Details and comments

In the future, I believe the VariationalResult class should be reviewed, and potentially include this ansatz property. But so far the eigensolver result does not inherit from VariationalResult, and for the sake of including this fix into the upcoming 0.22 release, I have decided to add it to the Eigen-results directly.

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[coveralls]

Pull Request Test Coverage Report for Build 3161882352

• 21 of 21 (100.0%) changed or added relevant lines in 4 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.003%) to 84.654%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%
qiskit/pulse/library/waveform.py	3	91.49%

Totals
Change from base Build 3161881839:	-0.003%
Covered Lines:	61498
Relevant Lines:	72646

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💛 - Coveralls